1. Field of the Invention
This invention relates to leucite chemically derived from a stable dispersion of a potassia precursor, an alumina precursor and a silica precursor. More specifically, it relates to congruently crystallized tetragonal leucite useful as a component of a dental porcelain. A process of preparing chemically derived leucite is also described.
2. Description of the Background Art
Porcelains are widely used in the dental industry as tooth restorations. A common type of restoration is a metal-based restoration, wherein several layers of porcelain are sequentially fused to a metal framework generally referred to as a "coping". These dental porcelains are typically blends of many frits or components. Various frits are combined to achieve certain desirable properties, such as color, strength, translucency, shock resistance and coefficient of thermal expansion ("CTE").
Conventionally, dental porcelains are made by admixing a glass frit (having a low CTE and optionally containing pigments and fluorescent additives) and a leucite-containing frit (having a high CTE) in appropriate ratios. The leucite-containing frit is derived by processing (i.e., firing) naturally occurring minerals such as potash feldspar (KAlSi.sub.3 O.sub.8), albite feldspar (NaAlSi.sub.3 O.sub.8) or nepheline syenite (NaKAlSi.sub.3 O.sub.8). In order to be useful as a component in porcelain, these feldspathic minerals are typically purified to remove contaminants, especially undesirable quantities of transition metal cations such as iron, chromium, nickel and manganese.
An important criterion in selecting a dental porcelain is the degree to which the CTE of the porcelain and that of the substrate or coping match. It is generally desirable to utilize a dental porcelain with a CTE slightly less than the CTE of the metal coping such that, upon cooling, the porcelain is placed in compression. Another important criterion in selecting a dental porcelain is translucency, which is directly proportional to the amount of crystalline phase in the porcelain. The degree of light transmission through a porcelain is directly dependent on the difference in refractive indices of the respective components, the volume fraction crystallized ("Vc") of leucite and the amount of leucite in the final composition.
U.S. Pat. Nos. 3,052,982 and 3,052,983 to Weinstein disclose mixing glass and glass-ceramic frits of different composition and different CTE. The resultant porcelains exhibited an intermediate CTE and the requisite mechanical integrity. By combining frits of various melting temperature, composition, and CTE characteristics, the user could tailor the CTE of the porcelain to match that of the desired metal base. After processing, the porcelain consisted of a glassy matrix with integrated tetragonal leucite.
It was recognized very early in the dental porcelain industry that tetragonal leucite, in particular, was a critical component in glass-ceramics to obtain the correct thermal expansion matching. Furthermore, its presence in dental porcelain was recognized as advantageous because it could impart higher strength, greater durability and the desired translucency to the final porcelain. Therefore, considerable effort in the field of dental porcelains has been directed to mixing as many as seven frits, each processed under different conditions, to obtain a final fired porcelain containing some tetragonal leucite phase and exhibiting the overall desired chemical, mechanical and aesthetic characteristics.
U.S. Pat. No. 3,464,837 to McLean et al. discloses a dental material formed from naturally occurring materials such as refractory oxides and feldspar. The material is said to exhibit superior mechanical strength.
U.S. Pat. No. 4,604,059 to Klaus et al. discloses blends of two different glass frits with two different glass-ceramic powders in varying ratios. The resultant blends exhibit thermal expansion coefficients varying from about 10.times.10.sup.-6 /.degree. C. to about 19.times.10.sup.-6 /.degree. C. The mineral feldspar is one of the components.
Work has also been directed to finding a single frit which would meet the thermal expansion coefficient requirements for dental porcelains. For example, U.S. Pat. No. 4,455,383 to Panzera discloses mixtures containing silica, alumina, potassia, a fluxing agent and other metal oxides to obtain a frit having a fired CTE of about 12-14.times.10.sup.-6 /.degree. C. With proper heat treatment, the crystalline phase leucite was formed. Rouf et al. in Trans. Br. Ceram. Soc. (1978), Vol. 77, p. 36 describe work directed to controlling the crystallization of aluminosilicate glasses by means of different nucleating agents. The goal was to produce a glass-ceramic with the desired crystalline phase.
U.S. Pat. No. 4,798,536 to Katz discloses the addition of potassium salts to feldspar to produce porcelains having a greater amount of leucite phase and improved strength.
U.S. Pat. No. 5,071,801 to Bedard et al. discloses a process of ion exchanging a zeolite to obtain a ceramic article having tetragonal leucite as the principle crystalline phase. Thermal expansion of the material is controlled by introduction of a pollucite (CsAlSi.sub.2 O.sub.6) phase into the leucite.
Limitations of conventional processing methods include the necessity of attaining high fusion temperatures as well as inhomogeneity of the processed powders. In contrast, sol-gel chemical ceramic processes are recognized as a way to obtain high purity, homogeneous ceramic compositions which require lower fusion temperatures for densification. These methods have been applied to the production of multicomponent feldspathic glasses. Jones et al. in J. Canadian Ceramic Society (1986), Vol. 55, p. 42-49 describe the cohydrolysis of aluminum alkoxides and ethyl orthosilicates in the synthesis of multicomponent feldspathic glasses using alkaline alkoxides and colloidal solutions of silica and alumina. For example, Rizkalla et al. in Br. Ceram. Trans. J. (1991), Vol. 90, p. 81-84 report the use of metal alkoxide hydrolysis to produce eight-component feldspathic glasses. These methods require the use of fluxing agents, nucleating agents and grain growth inhibitors to obtain a glass with the desired characteristics.
C. P. Mabie and coworkers have reported on a gel route of preparing low fusing dental porcelain frits in J. Biomedical Materials Research (1983), Vol.17, p. 691-713. Chloride stabilized alumina and alumina-silica sols were used. U.S. Pat. No. 4,431,451 to Mabie et al. discloses a dental porcelain frit prepared by a gel route.
Emu et al in JP 5-43312 describe the formation of anorthite (CaOAl.sub.2 O.sub.3 2SiO.sub.2) feldspathic powders for low cost substrates with improved purity and lower sintering temperatures. Emu utilized a boehmite (AlOOH) sol, a colloidal silica sol and a calcium salt which were gelled by boric acid and sintered at temperatures of 900.degree.-1200.degree. C.
Typically, leucite is incongruently crystallized (i.e., the stoichiometric composition of the amorphous matrix differs from that of the crystalline phase which derives from it) by the heat treatment of a precursor (i.e., antecedent) glassy matrix containing potassia, alumina, silica and other components such as alkali fluxes, nucleating agents and grain growth inhibitors. Feldspathic minerals, which include potash feldspar, albite feldspar, and nepheline syenite, usually provide the requisite precursor oxide matrix. To crystallize a significant percentage of leucite from a potassium aluminosilicate mineral, a minimum of about 12 weight % potassia is required. These conventional processes require additional alkalies (e.g., Li.sub.2 O, Na.sub.2 O, or K.sub.2 O) as fluxing agents to reduce the liquidus temperature of the parent mineral source. Even when fluxing agents are used, the firing of feldspathic minerals typically exceeds 1100.degree. C. and usually requires temperatures in excess of 1200.degree. C. for several hours. Additionally, nucleating agents (e.g., CaO, ZrO.sub.2, TiO.sub.2) are generally added to initiate incongruent crystallization of leucite. The glass-ceramics obtained by this method generally contain less than about 40 volume % tetragonal leucite with the residual glass matrix varying in composition based on the amount of leucite crystallized.
Leucite, which has the chemical composition K.sub.2 OAl.sub.2 O.sub.3.4SiO.sub.2, can manifest either a tetragonal or cubic crystalline phase. High temperature frit formation can produce varying amounts of the cubic phase of leucite, a metastable phase which usually converts to the tetragonal phase upon cooling. The cubic phase has a low CTE (approximately 10-12.times.10.sup.-6 /.degree. C.) and its presence in dental porcelain is particularly undesirable because of the low CTE and the volume expansion accompanying the cubic to tetragonal transition upon cooling during preparation of a porcelain fused to metal restoration. The tetragonal phase, on the other hand, has a high CTE (approximately 22-30.times.10.sup.-6 /.degree. C.) and is very desirable for admixture with glasses which generally have a low CTE and for bonding with metal substrates having a high CTE. Therefore, much work has been directed to controlling the formation of tetragonal leucite and its subsequent phase purity in porcelains, generally by controlling the time-temperature conditions under which the porcelain is processed.
The importance of using tetragonal leucite has been recognized in the dental porcelain art for some time. However, none of the aforementioned processes provides chemically derived leucite or congruently crystallized leucite.